Wikipedia:Reference desk/Archives/Science/2014 August 19

= August 19 =

Lowest Possible Temperature
We know there is a lowest possible temperature, but is there a theoretical highest? KägeTorä - ( 影 虎 ) ( Chin Wag )  01:18, 19 August 2014 (UTC)
 * WHAAOE. Absolute hot.  -- Jayron  32  01:20, 19 August 2014 (UTC)
 * There's also negative temperature. Dmcq (talk) 16:09, 19 August 2014 (UTC)


 * So, if a system at negative temperature is brought into the presence of a system at absolute hot, which way will heat flow? The article says " If a negative-temperature system and a positive-temperature system come in contact, heat will flow from the negative- to the positive-temperature system." - but I have no idea if that applies to absolute hot... SemanticMantis (talk) 16:52, 19 August 2014 (UTC)

Atoms as fact
When did the scientific consensus become that atoms are real? I read atomic theory, but it doesn't mention any point in time or event. Bubba73 You talkin' to me? 04:20, 19 August 2014 (UTC)
 * See there about Dalton in the section 'First evidence-based theory' and his presentations of the atomic theory at the start of the nineteenth century. His first oral presentation in 1803 is one good date and his textbook in 1808 is another. Dmcq (talk) 08:03, 19 August 2014 (UTC)
 * The term "atom" or "indivisible particle" was revived 1805 from ancient Greek philosophical speculation by John Dalton. In 1828 "atom" was evidently part of Christian truth with which Noah Webster was concerned in his dictionary. Understanding of the atom has progressed from the cubic model (1902), the plum-pudding model (1904), the Saturnian model (1904), and the Rutherford model (1911) to the Rutherford–Bohr model or just Bohr model for short (1913) whose image is most recognized today. The Bohr model is widely taught as real and gives an explanation of some spectral emissions. It may be said to have reached popular consensus though scientific consensus favours the quantum mechanical atomic model introduced by Wolfgang Pauli in 1925. 84.209.89.214 (talk) 10:41, 19 August 2014 (UTC)
 * First of all, scientists don't think in terms of "facts" and "proof". The think in terms of "evidence".  Dalton's atomic theory was well a well accepted conjecture by the middle 19th century, but the definitive empirical justification ("proof" if you want) is usually cited as Einstein's paper on Brownian motion in 1905 (his Annus Mirabilis.  See Annus Mirabilis papers).  The other parts of atomic theory, those dealing with the internal structure of atoms, built up over time, with the experiments and conjectures cited above by 82.209.89.214, who did a great job laying out the history of Atomic theory.  There were also some false starts in Atomic theory, see Prout's hypothesis for a more famous one.  -- Jayron  32  14:09, 19 August 2014 (UTC)


 * That is kind of what I was getting at. The 19th century chemists had a model that worked beautifully, but I don't think any of them "detected" an atom.  Bubba73 You talkin' to me? 02:56, 20 August 2014 (UTC)


 * I'm sure I have read somewhere that atoms were widely accepted among scientists as a model by 1905 but there were still some who didn't. Einstein's Brownian motion paper which provided another way to calculate the size of atoms was an extra bit of useful evidence that atoms existed and weren't just an apparent building block of solids that was actually made up of other things of different sizes. (oops should have read Jayron's answer first)  JMiall  ₰  18:53, 19 August 2014 (UTC)


 * There is a subtle confusion here. Once the experimental results were in and the math had been crunched, it would have been valid to say "The world operates as if matter was made of atoms" - but you still wouldn't know that there actually ARE atoms.  These days, we've actually imaged them so we have direct evidence - but that's a rather recent thing. SteveBaker (talk) 05:24, 20 August 2014 (UTC)

Can you accelerate the decay of radioactive material?
Or do you always have to bury it and wait 10,000 years? OsmanRF34 (talk) 15:10, 19 August 2014 (UTC)


 * For practical purposes, decay rates for spontaneous radioactive decay are constant. Radioactive decay says "The radioactive decay modes of electron capture and internal conversion are known to be slightly sensitive to chemical and environmental effects that change the electronic structure of the atom, which in turn affects the presence of 1s and 2s electrons that participate in the decay process. A small number of mostly light nuclides are affected ... A number of experiments have found that decay rates of other modes of artificial and naturally occurring radioisotopes are, to a high degree of precision, unaffected by external conditions such as temperature, pressure, the chemical environment, and electric, magnetic, or gravitational fields". Gandalf61 (talk) 15:22, 19 August 2014 (UTC)


 * The key words here are *spontaneous* radioactive decay. While temperature, pressure, electromagnetic fields, etc. don't substantially increase decay rate, neutron flux will. See Nuclear transmutation for details. -- 160.129.138.186 (talk) 16:21, 19 August 2014 (UTC)


 * Well surprisingly you can just about do the opposite see Quantum Zeno effect. And of course sending particles at something will start splitting it up, a nuclear bomb is a good example. Dmcq (talk) 16:02, 19 August 2014 (UTC)


 * You can sort of do that partly, by refining the material aka split it up into active and decayed matter with physical and/or chemical methodes of process engineering. This is actually done to an limited extend for some Time with radioactive waste because in most countries space in radioactive waste repositories is very expensive. --Kharon (talk) 16:35, 19 August 2014 (UTC)


 * You should be able to change a radioactive isotope into something else by bombarding it with enough of the right type of radiation. However, it may turn into something even nastier.  Also, you are likely to get many different products, so now you need to separate the stable from the radioactive.  There's also the risk that you might get more heat than anticipated, due to unknown interactions between all the products and radiation, and have an explosion or meltdown. StuRat (talk) 16:56, 19 August 2014 (UTC)


 * If you want to accelerate the decay, that means you want something "even nastier". Things that decay slowly are not very radioactive. But as Stu says, the problem is that you are likely to get many different products.  I don't believe it's practical at present to convert all of a chunk of some substance into another by irradiating it. --50.100.184.117 (talk) 00:24, 20 August 2014 (UTC)


 * It is possible using fast breeder reactors, but the anti-nuclear movement has stopped the development of large scale nuclear power making fast breeder reactors uneconomical at present. The Integral fast reactor was never completed, the article says: "Fast reactors can "burn" long lasting nuclear transuranic waste (TRU) waste components (actinides: reactor-grade plutonium and minor actinides), turning liabilities into assets. Another major waste component, fission products (FP), would stabilize at a lower level of radioactivity than the original natural uranium ore it was attained from in two to four centuries, rather than tens of thousands of years." Count Iblis (talk) 17:34, 19 August 2014 (UTC)


 * Giant resonance does not need fast breeding reactors or other sources of neutrons. But probably prohibitive amounts of energy. 95.112.198.190 (talk) 19:12, 19 August 2014 (UTC)


 * Another option is to separate out the radioactive isotopes from the non radioactive material. This my make a valuable resource, or at the least make a smaller waste output. Graeme Bartlett (talk) 23:10, 19 August 2014 (UTC)


 * What if extremely strong magnetic fields are used to deform the core atomic orbitals? Will that not affect the stability of the nucleus toward certain types of decay? Plasmic Physics (talk) 00:16, 21 August 2014 (UTC)


 * Interesting. I wonder if the field strength in a magnetar would be enough to affect nearby atoms (I say "nearby", because the magnetar itself, being a type of neutron star, is not made of complete atoms, at least not in the core). StuRat (talk) 22:19, 22 August 2014 (UTC)

Decline of large aerial predators
The amount of large aerial predators seem to be nonexistent, which is counter-intuitive to me. Considering there used to be creatures with wingspans the width of a basketball court, what led to their extinction? Considering there are still whales and elephants, it seems weird to me that bustards are the largest bird left around, with no large bird-of-preys. Was it tied to lowered oxygen levels? Climate change affecting updraft? Trees growing taller to hide prey? Just curious. --Wirbelwind( ヴィルヴェルヴィント ) 19:26, 19 August 2014 (UTC)


 * No large birds of prey? Tell that to the harpy eagle or the golden eagle. Here's a nice video about the harpy eating monkeys, and here's one about the golden hunting wolves . As for the biggest flyers ever, what do you know of with a ~50 ft wingspan? (the width of a basketball court). Pterosaur_size tops out at ~35 feet, according to our article. The largest bird we have evidence of is currently Pelagornis_sandersi, which, with a wingspan of ~20 feet is admittedly ~2x the wingspan of our current leader, the wandering albatross. But enough with questioning your premise:
 * You're right that were some larger flyers in the past, but there were also lizard type things the size of school buses, and they went away too, most likely due to the Chicxulub impact. So the pterosaurs et al. went extinct due to massive disruption of the food chain, as all photosynthesis on earth was severely compromised for a long time (more info at dinosaur extinction). After that, everything was much smaller, and there was a lot of value on the R side of R-K selection. Now, we might ask the question, "why have no very large flying predators evolved again?" I think the key here is that size is relative. We lost most of our megafauna in the Holocene extinction, and even though that was pretty recent, things have pretty much stayed smaller since then. I don't personally think O2 levels or updraft patterns are the culprit, but that's hard to find references to support. Even though the Square-cube_law has never changed, I think its importance might be strengthened in a world with much more pathogens, complex food webs, greater diversity of terrestrial vertebrates, etc. The main thing to think of is the ecological niche, and, for whatever reasons, the niche space is just not there anymore, even if it used to be. SemanticMantis (talk) 20:13, 19 August 2014 (UTC)
 * I wasn't aware of golden eagles hunting wolves, but that does seem like a partially human-influenced behavior. Looking at the statistics in the article, the most commonly hunted canid is the fox, which still comprises of a tiny percentage. As for the 50' wingspan, blame National Geographic (or similar) for that figure. It was compared to a basketball court in a documentary. As for everything shrinking, I'm aware of that. I just meant it's counter-intuitive (maybe only) to me that aerial predators haven't thrived due to a lot of advantages flight should bring. --Wirbelwind( ヴィルヴェルヴィント ) 22:11, 20 August 2014 (UTC)


 * A giant asteroid slammed into the Gulf of Mexico some 65 million years ago, setting off a chain of catastrophic events that ultimately led to the extinction of all big living creatures. Then mammals got bigger. And eventually they paid the price. Several mammoths and other big mammals died off during the Pleistocene/Holocene extinction event, which started around 50,000 years ago. whales were more mobile and could emmigrate to whenever they wanted. Animals bigger than elephants didn't have so much luck. If there is little food, you are better off being small and having lots of descendants as fast as possible. OsmanRF34 (talk) 20:19, 19 August 2014 (UTC)
 * Thank you. This is more or less the kind of answer I was looking for. --Wirbelwind( ヴィルヴェルヴィント ) 22:13, 20 August 2014 (UTC)


 * Per Semantic Mantis, there are several birds of prey which can be strikingly large; it's just that humans rarely get up close to them to see them. All animals (aside from whales) are generally smaller than they were in the Mesozoic era and flying critters are no exception.  However, that doesn't mean that all birds are pigeon-sized.  Female bald eagles have a length of 3.5 feet or so (standing on the ground they'd come up past your belly button) and have a wingspan of 7.5 feet or more.  The golden eagle noted by Semantic Mantis above can get even larger.  Those are land-based raptors.  Seabirds can also get pretty large; various kinds of pelicans can get as large as a person; 5+ feet in length with an over 8 foot wingspan.  Obviously, they weigh less than people, but they are still quite big in size (bird physiology maximizes their ability to fly, and so tends to favor lower mass-to-size ratios than land animals).  The grand-daddy of carnivorous birds is the California condor,  which is a bit shorter than the largest pelican, but can have up to a 10-foot wingspan.  Again, since most people never see one of these birds up close, it can be hard to judge them from context.  If you ever do get to see one up close, it takes you aback at how big they can be.  -- Jayron  32  22:00, 19 August 2014 (UTC)
 * When it comes to pterosaurs, this is an open question. There is even a question about the question itself. Based on the fossil record, it appears that the pterosaurs reached peak diversity in the early Cretaceous period and then sharply declines before being finished off by the K-Pg extinction. However, it's possible that at least some of this apparent decline may be due to gaps in the fossil record . It should be noted that as far as we know, there were never any large, predatory pterosaurs (the thing with pterodactyls carrying prey off to their nest is a Hollywood invention;they didn't even have grasping feet, or bird-like nests). Why a highly diverse group like pterosaurs never produced any predatory forms is an interesting question and I'm not sure if it's been addressed. There may have been small hawk-sized predatory pterosaurs (this has been suggested for Darwinopterus), but even the biggest ones like azhdarchids seem to have been stork analogues, picking off small prey like lizards and baby dinosaurs. I guess that's still predatory, but maybe not the way most people think.
 * It's true that birds have not yet ever attained sizes comparable to pterosaurs and are on average very small (pterosaurs were on average very big as adults with no known tiny species, probably because they took years to grow up unlike birds, and niches for small sized animals were occupied by babies of larger species). It has been suggested that birds are limited in size by their takeoff style. They need two well-developed sets of limbs - forelimbs for flight, and hind limbs for launching themselves into the air. That's two well-developed sets of muscles in creatures that need to stay as light as possible. Pterosaurs were quadrupeds and most had piddly little hind limbs. By all accounts, they both launched AND flew with the forelimbs, allowing the biggest pterosaurs to get a few times bigger than comparable birds.
 * I don't think there have ever been any really large aerial predators. The largest I can think of is Haasts eagle, which isn't as large as the largest flying birds today, though larger than the extant predatory ones. All birds that ever lived larger than this have either been vulture-type birds (scavengers) or seabirds, which eat fish and can get larger than normal by taking advantage of oceanic winds to aid flight. Dinoguy2 (talk) 23:23, 19 August 2014 (UTC)
 * 'Large, flighted, actively-hunting bird' is probably one of the most energy intensive animal 'designs' possible. Imagine how much food a 150lb+ bird that chased down and subdued live prey would need to eat in order to remain alive, let alone thrive, given that flying birds have a much faster metabolism than mammals (from what I've seen, birds tend to eat a huge amount for their size) - and how much territory it would need to fulfil its hunting needs. Such a critter would certainly find it difficult to survive and almost certainly be amongst the first to die out, should the world ever take a turn for the worse. The condors are huge and carnivorous, to be sure - but they are also mostly scavengers and gliders. --Kurt Shaped Box (talk) 23:36, 19 August 2014 (UTC)

This is not a scientific answer because there is lack of scientific evidence for it. But I believe that not only is the percentage of oxygen higher in the past but the air pressure is higher in the past as well. In fact I believe the air pressure is 10 times to 50 times higher in the past than the air pressure is today. Therefore with higher air pressure in the past, flying creatures can be bigger because it can generate bigger lifts. Of course, air pressures leaves no "fossil records" and thus I am not able to offer any proof that air pressure is higher in the past. 202.177.218.59 (talk) 00:56, 20 August 2014 (UTC)
 * "This is not a scientific answer because there is lack of scientific evidence for it." So why post it? The OP asked a scientific question, so he presumably was not looking for weird beliefs based on fantasy as an answer. Might as well answer with "Well, in Middle-Earth, the air might have been like this..." People often feel the need to explain giant Mesozoic animals by invoking air density, air pressure, or differences in gravity, simply because they don't understand that the biology of these animals was different from any living today. We do not have any quadrupedal fliers with extensive pneumatic lungs today, because they were all killed by an asteroid. We don't have any land animals with extensive air sacs today either. If mammals had air sacs and R-selection breeding strategies, they could easily reach or exceed the size of dinosaurs, but they don't. No need to call on strange differences in air pressure or gravity with no evidence when we have plenty of evidence pointing to simple biological explanations. Dinoguy2 (talk) 13:28, 21 August 2014 (UTC)
 * We do not currently have a Thick atmosphere theory article, and Atmosphere of Earth does not address pressure. (Faint young Sun paradox does discuss the posibilities of CO2 partial pressures as high as 10 bar, but long, long before the cambrian radiation, presumably only up until the GOE.)  Here are a couple of interesting papers by Octave Levenspiel published in 2000 in ACS's Chemical Innovation: Earth’s atmosphere before the age of dinosaurs (with Thomas J. Fitzgerald & Donald Pettit, on the ACS site) and Atmospheric Pressure at the Time of Dinosaurs (on Levenspiel own site). -- ToE 05:01, 20 August 2014 (UTC)


 * According to Quaternary extinction event and given the many near extinct species, caused by overhunting and -fishing done by humans up till today, its almost certain there where some but Homo sapiens sapiens killed them all.
 * Even worse. Your contradiction regarding whales and elephants is wrong. We (as species) would likely have killed all whales and elephants by now if some international Commissions had not put a world wide ban on commercial whaling and ivory trading. --Kharon (talk) 02:37, 20 August 2014 (UTC)
 * It's also interesting to note that the only continent left with an in-tact megafauna is sub-Sahara Africa, which also happens to be the only megafauna that co-evolved with humans, and presumably evolved strategies to avoid human predation. It was only after large scale access to killing using cars and guns that they went into steep decline. Dinoguy2 (talk) 13:39, 21 August 2014 (UTC)